1. Field of the Invention
The present invention relates generally to a device and method for varying coronary perfusion pressure to provide therapeutic assistance in cases of insufficiency of coronary circulation and depressed cardiac contractile function.
2. Description of the Prior Art
Approximately 1/2 million Americans die of acute heart failure annually. Of these deaths, approximately 50% occur in spite of medical treatment (the other 50% do not reach the hospital). Although acute heart failure is presently treated with drugs and other therapy, present interventions are not sufficiently effective. Clearly, additional measures are needed to help save lives of patients suffering from acute heart failure due to obstruction of the coronary vasculature or due to extensive cardiac surgery or other causes.
The coronary circulation system delivers blood to the heart muscle during the relaxation phase of cardiac contraction. During the contraction phase, pressure in the heart muscle rises and restricts coronary inflow, even though the arterial pressure rises due to cardiac ejection of blood. This elevation of coronary pressure increases the stiffness of the heart wall. With increased stiffness, the heart must expend more energy to bend the heart wall in order to eject blood. In other words, contraction of the heart against a large coronary pressure results in more "internal work" relative to the beneficial "external work" of ejecting blood from the heart chamber. This phenomenon has been postulated in the literature with reference to the "Gregg Phenomenon" and also with reference to the "garden hose" effect. The Gregg Phenomenon refers to the observation made in the 1950's by Donald Gregg, a noted American coronary physiologist, that myocardial oxygen consumption varied with coronary perfusion pressure, even when all other determinants of myocardial oxygen demand, such as heart rate and arterial pressure, were held constant. More recent studies have shown that the Gregg Phenomenon results from coronary pressure induced changes in the ventricular systolic stiffness. See, for example, "Coronary Perfusion Related Changes In Myocardial Contractile Force And Systolic Ventricular Stiffness", by Iwamoto, Bai and Downey, Cardiovascular Research, 1994; 28:1331-1336. It can be theorized that, when the systolic coronary pressure is decreased, the heart wall becomes less stiff and can be more readily deformed during cardiac contraction. Thus, with reduced coronary pressure during the cardiac phase of contraction, the heart muscle requires less oxygen to overcome this important component of "internal" cardiac work.
The garden hose effect refers to the erectile characteristics of the coronary circulation and parallels the analogy of the Gregg Phenomenon. Alteration of coronary pressure changes the amount of blood in the coronary circulation and affects its rigidity. During cardiac contraction when arterial pressure is highest, the coronary pressure is also high, and this tends to distend the coronary circulation as does turning on a faucet to a garden hose. The garden hose effect is thus directly related to the Gregg Phenomenon. For further background on the Gregg Phenomenon see "Coronary-Ventricular Interaction: The Gregg Phenomenon", by Downey, Cardiac-Vascular Remodeling and Functional Interaction, Ed. Maryama, Hori, Janicki, Tokyo, 1997, pp. 321-332.
The present invention has as an object to provide a method and device for varying or modulating coronary perfusion pressure according to the phases of a patient's cardiac cycle. Since perfusion pressure alters myocardial stiffness, changes in systolic stiffness should affect myocardial oxygen demand by changing the ratio of internal to external work. With a decreased systolic perfusion pressure, myocardial oxygen demands will be reduced, thereby permitting an increase in myocardial oxygen utilization efficiency.
Another object of the invention is to provide a coronary counterpulsation device which will limit the energy expensive component of internal work during cardiac contraction, so that under conditions of limited availability of cardiac energy, more of the energy can be devoted to the external work of pumping blood.
Another object of the invention is to provide a device and method which will reduce coronary pressure during the contraction phase and which will also increase coronary pressure and blood flow during the relaxation phase, when coronary flow normally nourishes the heart muscle.
Another object of the invention is to provide a device and method for increasing coronary flow during the relaxation phase of the cardiac cycle which thereby increases flow through primary as well as supplemental, i.e., collateral, blood vessels supplying tissue whose normal source of blood flow has been compromised by coronary artery disease or other causes.
Another object of the invention is to provide a device and method for significantly decreasing cardiac morbidity and mortality.
Another object of the invention is to provide a coronary artery counterpulsation device and method which compliments existing acute heart failure therapies.